The present invention relates to an ultra-high molecular weight polyethylene powder as a starting material for the preparation of polyethylene fibers or films having a high strength and a high elastic modulus. More particularly, it is concerned with an ultra-high molecular weight polyethylene powder superior in solubility to organic solvents, obtained by combining a specific catalyst with a specific polymerization process.
So-called ultra-high molecular weight polyethylenes extremely high in molecular weight, which is above one million, have been used as characteristic engineering plastics superior in impact resistance and abrasion resistance and having a self-lubricating property, in a wide variety of fields such as food machinery, construction machinery, chemical machinery, agriculture, mining industry, sports and leisure industry, including hoppers, silos, various gears, lining materials and ski linings.
Ultra-high molecular weight polyethylenes are much higher in molecular weight than general-purpose polyethylenes, so it is possible that there will be obtained a drawn product having unprecedentedly high strength and high elastic modulus provided a high orientation can be attained. From this point of view, various studies have been made for attaining a high orientation of ultra-high molecular weight polyethylenes. However, as compared with general-purpose polyethylenes, ultra-high molecular weight polyethylenes are extremely high in melt viscosity and so can hardly be subjected to extrusion in a conventional manner and it has been impossible to draw them into a highly oriented state.
In Japanese Patent Laid Open No.15408/81 there is disclosed a process in which a gel obtained from a solution (dope) of an ultra-high molecular weight polyethylene in decalin is drawn at a high draw ratio to prepare a fiber having a high strength and a high elastic modulus. However, the polymer concentrations in the dope adopted therein are extremely low, which are 3 wt.% and 1 wt.% in the cases of weight average molecular weights of 1.5.times.10.sup.6 and 4.times.10.sup.6, respectively. For practical use, a dope of a higher concentration is desired in view of problems such as the disposal of a large amount of solvent and economy.
The preparation of a polyethylene solution involves no special problem in the case where an average molecular weight of the polymer is about several hundred thousand or less. It can be effected easily by heating and agitating the dope at an appropriate temperature using a suitable solvent. But ultra-high molecular weight polyethylenes above one million in average molecular weight are extremely high in viscosity even in a solvated state. Even if the polymer particles are solvated and swollen at surface portions thereof, the particle surfaces are covered with the solvate to prevent permeation of solvent to the interior Further, there occurs fusion bonding between particles, which impedes a homogeneous dissolution.
For overcoming the above-mentioned difficulty of dissolution of ultra-high molecular weight polyethylenes and thereby preparing a higher concentration solution, there has been proposed, for example, a supersonic wave radiating method or a concentrating method from a dilute solution. However, both methods involve the likelihood of the molecular chain being cut and are therefore not preferable when it is intended to develop intrinsic properties of ultra-high molecular weight polyethylenes through a subsequent forming step.
Thus, it is by no means easy to dissolve an ultra-high molecular weight polyethylene in a solvent to prepare a homogeneous forming dope. This has been a serious obstacle to an industrial application of the conventional method.